Thumbnail too little | RSS Retriever | Support Forum

Actually it's quite logical that images are taken from the RSS feed, since the plugin is called RSS Retriever. However, RSS feeds can be very different. For example, images can be included not only in the post body, but also as media attachments. In addition, the plugin can retrieve the full content of the imported post by retrieving it from the original web page linked in the RSS feed. There are specific settings for all of these features.

I see you found that the retriever gets the image from the feed instead of the post. This makes sense given the functionality of the plugin. However, it's hard to give an accurate answer without a link to the RSS feed you're using, as it could contain different types of data that require different plugin settings.